Densenken-Lightning of a Thousand Fists.

[Spokavriel]

The results are as varied as the materials you can use to build buildings. Anywhere from no reaction to pulling the building around. And if there's a frequency to it you could even heat some structures with magnetic fields. But try it on a classic construction Pagoda (Held together with pegs, glue and wooden joinery) nothing will happen.

[Knight of L-sama]

I would point out there's the issue of mass to contend with. Most buildings that use a lot of steel (either steel frame or steel-re-enforced concrete) tend to be... large. A small object generating a magnetic field is going to be pulled towards the building rather than pulling the building towards it.

[Ellen Kuhfeld]

If you have a powerful enough magnet, you can levitate things like acorns, grasshoppers, and frogs. (I've seen the videos. Small frogs.) They haven't made magnets that powerful that can handle larger things. So if it were a very small building you might have luck. And MRI scanners are powerful magnets -- there better not be any iron in the room, or nearby rooms.

The best way for a large magnet to affect a building? Drop it on the building.